Will David Cameron be Britain's first Euro-sceptic Prime Minister?

Full coverage of UK PoliticsDavid Cameron has just confirmed that, if ratification of the European Constitution Lisbon Treaty were not complete in every one of the 27 EU states, an incoming Conservative Government would carry out its promise to hold a referendum and recommend a "No" vote.

David Cameron speaks to Andrew Marr

The Euro-phile Andrew Marr assumed an expression of almost Paxmanesque incredulity, and you can see why. The prospect of a Conservative administration nullifying the Constitution, just when every other state had adopted it, horrifies Eurocrats. As one of them has told The Economist, "nuclear is not a strong enough word".

Until now, Euro-integrationists have reassured each other that Cameron doesn't mean it, that he is simply making populist noises while in Opposition. Their confidence is understandable. Regular readers will be familiar with Hannan's First Law: that no party is ever Euro-sceptic while in office. Even so, the unequivocal, almost swaggering, way in which Cameron keeps repeating his promise is alarming even the most complacent of Brussels functionaries.

In a related demonstration of the rule, the Polish President, Lech KaczyÅ„ski, having dropped his opposition to the Constitution in return for an aid package to the Gdansk shipyards, is now reported to be trying to persuade his Czech counterpart, VÃ¡clav Klaus, also to back down. Klaus has already demonstrated Hannan's First Law: when he was Prime Minister during the 1990s, he dropped his opposition to both the euro and the Amsterdam Treaty, only rediscovering his scepticism when he resigned.

So it will all come down, as usual, to Britain. There will be immense pressure on a newly elected Cameron to defer Europe to a second term, to concentrate on mending Britain's broken society, not to pick too many fights at once. But, so far, he shows no sign of weakening.